The present invention relates to a digital signal reception system, in particular, relates to such a system and which provides a superior error rate performance under a low S/N transmission path corrupted by high power and narrow band interferences within the digital signal frequency band.
To improve the S/N (signal-to-noise ratio) of the digital signal, a prior digital signal reception system is composed as shown in FIG. 1. In FIG. 1, the digital radio signal at the input terminal 100 is applied to the bandpass filter 1, which passes the whole frequency spectrum of the digital signal and eliminates the thermal noise and the interference noise outside the passband. Thus, the signal-to-noise ratio of the digital signal is improved by the bandpass filter 1. The demodulator 4 is connected to the output of the filter 1, and said demodulator 4 provides the demodulated baseband signal. The timing signal regenerator 5 recovers the timing signal from the demodulated baseband signal, and the regenerator 6, which is connected to the output of the demodulator 4, regenerates the digital signal by using the timing signal from said timing signal regenerator 5. Thus, the regenerated digital signal is obtained at the output terminal 101 connected to the output of the regenerator 6.
However, said prior digital signal reception system has the disadvantage that the S/I (signal-to-interference power ratio) can not be improved when an interference noise (N) exists within the digital signal frequency band (D) as shown in FIG. 2(A). Therefore, in order to obtain the required S/N or S/I, the receiving power must be increased by shortening the hop distance, and/or increasing the transmitting power. However, those techniques increase the number of repeater stations, the size of the radio set and the power consumption of the set, and degrades the reliability of the apparatuses.
When a digital radio system is newly constructed in the same radio frequency band and in a different route with existing analog FDM-FM systems, the digital radio system should be designed as follows. First, the intersystem interference from the digital system to the existing FM system has to be reduced to less than the allowable limit of the FM system. Secondly, the required C/I (carrier to interference power ratio) of the digital system has to be satisfied.
The relationship of the spectrum between the digital system and the analog FM system is shown in FIG. 2B, where D is the spectrum of the digital circuit. It should be noted that the analog FM signal of an interference gives almost the same effect to the digital system as in the case of FIG. 2A. In FIGS. 2A and 2B, the center frequency of the digital radio signal and the interference signal is f.sub.O, and f.sub.O +f.sub.I, respectively.
When a new digital radio system is constructed in the same frequency hand and in different route with an FM system, an antenna with excellent directivity has to be employed to reduce the intersystem interference. Alternatively, the repeater station site for the digital circuit has to be selected so that the incident angle of the analog FM circuit to the antenna of a digital circuit is large enough in order to attenuate the analog FM power by antenna directivity. If the site selection is restricted and the incident angle to the antenna between the analog and the digital circuits is not large enough, the transmitting power of the digital system must be sufficiently increased to provide the required signal-to-noise ratio.
As mentioned above, it becomes difficult to newly construct a digital system under the co-existance condition with a conventional FM system, because site selection for the digital system and/or the transmitting power of the digital system are significantly limited.